1. Field of the Invention
The present invention relates to a rolling bearing, and more specifically, it relates to a rolling bearing suppressing occurrence of seizure.
2. Description of the Background Art
Following the recent improvement in performance and efficiency of a mechanical device employing a rolling bearing, high durability in a severe environment tends to be required to the rolling bearing. More specifically, a rolling bearing employed in a contaminated environment penetrated by hard foreign matter may be damaged in an early stage (in an operating time shorter than a calculated life of the bearing) due to gripping of the foreign matter. On the other hand, a rolling bearing used in an insufficiently lubricated environment may cause seizure. Such seizure is easily caused particularly between a cage constituting the rolling bearing and a race member such as an inner ring or an outer ring guiding the cage. When the rolling bearing is used in a high-temperature environment of a temperature exceeding 200° C., for example, hardness of components such as the race member and rolling elements constituting the rolling bearing may be reduced, to reduce the durability of the rolling bearing.
When the components such as the race member and the rolling elements are made of steel, strength at a high temperature can be improved for improving the durability of the rolling bearing in the high-temperature environment by adding at least 3.75 mass % of chromium to the steel thereby improving tempering softening resistance of the steel. In order to improve the durability in the contaminated environment, a treatment of forming nitrogen-enriched layers having higher nitrogen concentrations than the remaining regions on surface layer portions of the components such as the race member and the rolling elements by performing nitriding, for example, can be employed.
In components made of steel having a high chromium content of at least 3.75 mass %, for example, chemically stable oxide films are formed on the surface layer portions. When ordinary nitriding is performed on these components, therefore, nitrogen does not penetrate into the surface layer portions thereof, and no nitrogen-enriched layers are formed. In relation to this, there is proposed a countermeasure of forming nitrogen-enriched layers by performing plasma nitriding (refer to Japanese Patent Laying-Open No. 2-57675 (1990: Patent Document 1), for example). As to control of the plasma nitriding, there are proposed a method based on spectral analysis of glow discharge and a method based on the density of a current flowing in treated objects (refer to Japanese Patent Laying-Open No. 7-118826 (1995: Patent Document 2) and Japanese Patent Laying-Open No. 9-3646 (1997: Patent Document 3), for example). Thus, nitrogen-enriched layers can be formed on the surface layer portions of the components such as the race member and the rolling elements made of the steel containing at least 3.75 mass % of chromium.
In order to improve seizure resistance, there is proposed a countermeasure of dipping balls serving as the rolling elements in an organic phosphorus compound for forming reaction layers on the surfaces thereof (refer to Japanese Patent Laying-Open No. 9-133130 (1997: Patent Document 4), for example).
Even if the nitrogen-enriched layers are formed on the surface layer portions of the components such as the race member and the rolling elements made of the steel containing at least 3.75 mass % of chromium by the aforementioned plasma nitriding, however, the characteristics of the components may not be sufficiently improved. In other words, separation or fracture may be caused in an early stage when stress is repetitively applied to the aforementioned components (reduction in fatigue strength). Further, breakage may be easily caused when impactive stress is applied to the aforementioned components (reduction in toughness). Thus, sufficient characteristics may not necessarily be attained particularly in fatigue strength and toughness when merely nitrogen-enriched layers are formed on the components such as the race member and the rolling elements made of the steel containing at least 3.75 mass % of chromium, although the surface layer portions thereof are improved in hardness.
The environment in which the rolling bearing is used is becoming increasingly severe. To a rolling bearing used for a jet engine of an aircraft or the like, for example, not only suppression of reduction in hardness of components in a high-temperature environment and improvement in durability in a contaminated environment but also improvement of seizure resistance in a case where lubrication is temporarily stopped (improvement of the so-called dry-run performance) are required. In order to improve the dry-run performance, seizure between a race member and a cage must be efficiently suppressed, in particular. Therefore, conventional countermeasures including those disclosed in the aforementioned Patent Documents 1 to 4 cannot necessarily be regarded as sufficient.